A switch capacitor power converter, also known as a charge pump, is a circuit that converts a supply voltage Vdd to an output voltage Vo, that provides power to a load using switches and capacitors. The switches are usually semiconductor switches such as diodes, NMOS (n-type metal oxide semiconductor) or PMOS (p-type metal oxide semiconductor) transistors. The conversion ratio M is defined as the ratio of the output voltage Vo to the supply voltage Vdd, that is, M=Vo/Vdd. Charge pumps that have an output voltage Vo higher than the supply voltage Vdd are step-up charge pumps.
Step-up charge pumps have found applications in pacemakers where a high voltage is needed to stimulate the heart muscle using only a one-cell battery that has a low voltage. The high voltage may be 30V and the battery may have a voltage of 1.2V, giving a conversion ratio of M=25. They have also found applications in liquid crystal display drivers of handheld equipment. A liquid crystal display may need 20V to operate, but the handheld equipment is usually powered up by a battery of 3.6V. Hence, a step-up charge pump with a conversion ratio of 6 will be needed. In integrated circuit applications, an EEPROM may need a 12V supply voltage to perform programming and erasing of the programmable read only memory (PROM), but other circuits may use a 1V supply voltage. In this case, a single supply voltage of 1V can be used, and the voltage of 12V is generated by a charge pump with a conversion ratio of 12. To summarize, charge pumps with high conversion ratios are used in many practical applications.
Charge pumps of various different designs are well-known. Here, only a few popular designs that have earned industrial recognition are discussed. For easy comparison, a conversion ratio of M=16 with a supply voltage of Vdd=2V is considered. The output voltage is then Vo=32V. In practice, the output voltage cannot reach 32V due to losses, but for the sake of this discussion, the charge transfers are assumed to be lossless.
Linear charge pumps that use diodes to charge the capacitors are popularly known as Dickson charge pumps. A linear charge pump has many stages N. For each stage, the corresponding capacitor is charged up to a voltage Vcharge. The output voltage Vo is the sum of N such voltages and the supply voltage Vdd, that is, Vo=Vdd+NVcharge. Linear charge pump are so called because the output voltage bears a linear relation with respect to Vcharge. Dickson charge pumps use diodes as switches. A diode drop is 0.7V, and with a supply voltage of 2V, the useful voltage for charging the capacitors is only Vcharge=1.3V. For an output voltage of 32V, a 24-stage linear charge pump is needed that requires 24 diodes and 25 capacitors, plus 4 large transistors for a 2-phase non-overlapping clock. A simple calculation shows that the efficiency of the charge pump is only 64% at best.
By replacing diodes with diode-connected NMOS transistors, the efficiency may be even worse, because the threshold voltage of an NMOS transistor may be larger than 0.7V, and body effects may result in an even larger threshold voltage. Researchers have tried different gate driving schemes, such that no voltage is dropped across the NMOS transistors that serve as switches. For such a scheme, additional circuitry is needed to open and close all the switches completely, and a 15-stage linear charge pump is still needed to achieve a conversion ratio of M=16, and 19 large transistors and 16 capacitors are needed, plus additional smaller transistors to drive the gates of the large transistors.
Another approach makes use of cross-coupled doublers. A cross-coupled doubler generates an output voltage of 2Vdd from a supply voltage of Vdd using 3 capacitors and 8 switches. By cascading 4 such doubler stages, an output voltage of 32V can be obtained from a supply voltage of 2V. A total of 32 large transistors and 12 capacitors are used.
Existing charge pumps use many capacitors and switches to achieve a high conversion ratio. The large number of components required by the above three charge pumps are not favorable, and a more efficient scheme in reducing the number of components would be preferable.